A family of heterotrimeric nucleotide-binding proteins that bind to guanine (G proteins) transduces chemical and sensory signals across the plasma membrane by sequential interactions with receptor and second messenger-generating effectors. Because of the wide array of cellular processes that are mediated by G proteins, the study of G protein function and regulation is a significant area of research in the signal transduction field. There are reports containing suggestions of an important function for G protein at cellular locations other than the plasma membrane. Certain G proteins were detected at intracellular membranes, for example, the Golgi complex, whereas others associate with cytoskeletal structures, for example, microtubules and microfilaments. The mechanisms that govern the cellular destinations of G proteins and the relative proportions of G proteins that traffic to subcellular compartment are just beginning to be revealed.
G proteins are reported to couple the receptors for hormones or neurotransmitters to intracellular effectors such as adenylyl cyclase or phospholipase C. Twenty forms of the α-subunit of G proteins were identified and each is involved in the conveyance of multiple hormonal neurotransmitter signals from the outside of the cell to the effects that those hormones and neurotransmitters have on the inside of the cell.
G proteins may leave the membrane in response to neurotransmitter or hormone signals, but this has been very difficult to prove.
GFP, an autofluorescent protein of 238 amino acids, is a reporter molecule useful to monitor gene and protein expression and to observe the dynamics of protein movements within the living cell. Fusing GFP to another protein of interest allows time-course studies to be performed on living samples in real time. Accounts of GFP fusion proteins include receptors, secretory proteins, cytoskeleton proteins and signaling molecules. Presently, GFP fusion proteins are constructed by generating an expression construct that contains GFP fused in frame to either the N-amino or C-carboxyl terminus of the protein of interest. However, this attachment may alter the function of the protein fused with GFP consequently may not give results reflective of the natural state.